Method of forming sheet metal



Patented Mar. 5, 1946 v METHOD OF FORMING SHEET METAL Howard M. Jager,Oceanside, N. Y., assignor to Republic Aviation Corporation; acorporation 1 I of Delaware Original application November 464,357.Divided and this application June 30,

1943, Serialllo. 492,795

4 Claims. This invention relates to forming flat sheets of metal intovarious shapes by the employment .of methods involving a drawingorstretching action and has more particular reference to a methodinvolving provisions for anchoring or holding the work sheet during saiddrawing or stretching operation. This application is a division ofapplication Serial Number 464,357, filed November 3, 1942, which maturedinto Patent Number 2,378,482 on-June 19,1945. 7

The generalobject of the i vention is to provide a practical method of'draw-forming or sketch-forming sheet metal into hollow aircraftcomponents of curved shape such as metallic wing leading-edge sectionsand the like. a

A particular object is to provide a sheet metal forming method involvinga stretching operation, in which the work-holding means will apply aholding-pressure that will be entirely independent of the stretchingstrain or load, so that there will be no possibility of the sheetlbeingtorn by the machine or becoming released during the stretching orforming operation.

A more specific object is to provide a sheet met-v al forming method,.involving a drawing action, wherein the holding pressure, or grip, ofthe holding means on the work sheet can be accurately set to the desiredamount before the machine is operated and yet can be readily adjustedfor any desired load lying between the elastic limit and I the ultimatetensile strength of the work sheet; but which will nonetheless apply adifferential pressure on the held parts of the sheet and where.- by thestretching or drawing load applied will be progressively absorbed andmet by progressively increasing sheet-retaining forces.

A further particular object is to provide a method which will spread theholding grip over a much larger area of the work sheet than thesubsistent work holding means; and in which a, 1942, Serial No.

' l i oppo'sed edges thereof are 'clampingly held againstslippage-orrelease by the application of progressive, multiple clampingpressures inanag gregate alwaysexceeding the stretching force applied tothe bodyof the sheet between theclampinglyheld edges. F j

While it, is important to so preform and clamp:

' inglyengagethe opposed edges of the sheet that there will be notearing of the sheet during the final step of the instant method, itis.eqilally important that the opposed edges be held against thestretching operation and that only a minimum of the sheet be preformedfor the purpose of creating clamping'or engaging zones. i g

Several embodiments of the inventive objects and concepts aredescribedhereinafter and shown in the accompanying drawing, butit istoj beunderstood that the invention i limited in its embodiments only by thescope of the sub-joined c1 ims- 1 a In thedrawing Figure 1' isafragmentary end elevationofa sheet metal forming machine embodying apreall the acting surface of the work holding means,

though of this improved nature, will be incapable of forming weakeninpunctures or indentations or other deformations in the held part of thework sheet, and will'be perfectly smooth and ob.- vi'ate the. employmentof any kindof gripping teeth or projections.

In short this "invention resides in the'se'veral successive steps ormanipulations, hereinafter described in detail, for .the conv'ersion, ofa flat metal sheet into a hollow body, such as aircraft components, forexample, the leading edge of a wing and proposes'a method whereby theultimate form of the body is attained by stretching a preformed sheetbeyond its elastic limit While ferred form of apparatus'for carrying'outa part of the instant method; f S

Figure 2is a partial transverse section showing a modification of theclamping instrumentalities shown in Figure l;

Figure 3 is a force diagram showing the action of the forces in theclamp of Figure 2;and

Figure 4 is a fragmentary bottom plan viewof the clamp of Figure 2, withcertainlparts insection.

The forming machine shown in Figure 1 is there applied to form a skin ofa leading edge sec-- tion of an airplane wing and includes a stationaryform block H, carried on a platen l2 of a conventional hydraulic pressand a' saddle-shaped frame 13 to which the metal sheet is clamped andwhich frame is acted upon by the ram' (not shown) of the hydraulicpress. v A

The metal sheet Zlito be formed isbent beforehand to conform roughlywith the shape of the block it and has its oppositelongitudinal edgesbent twice in a brake so as to form two externally projectingU'-channelled retaining borders. Thus prepared, the sheet 20 is slid'endwise oyer the block H, while each ofsaid U-borders isin sertedbetween two longitudinal beams l4 and il forming the upper and lowerjaws of a clamp.

In the upper jaw M is provided a longitudinal dovetailed recess E5 inwhich works a longitudi nal wed -shaped sans nut l6; cooperating with aeries of screws 18 for gripping andanchorin hand threaded portions.

the U-shaped ends of the work sheet 20. One side [9 of the lower jaw llis slightly higher than the other to compensate for the thickness of thesheet 20. 7

When thus clamped in position over the form I I, the work sheet20 can bestretched by lowering the ram of. the press over the frame I3 until thesheet is deformed and assumes permanently the shape of the form. Duringthis operation, there is no risk of tearing the work sheet along theinner bend of the U-borders, even if the pressure exerted by the ramrises beyond the ultimate tensile strength of this sheet, due to thefact that the gripping pressure is independent of the stretching forceand is adjusted by tightening the screws l8 to the desired value lyingbetween the elastic limit and the tensile strength of the sheet, andalso due to the fact that the gripping pressure is spread over arelatively large area of the sheet, i. e., the wing of the U-border ispressed between the wedge 15 and one slanting side of the recess I 5 andthe bottom of the U-border is pressed between the two jaws l4, l1, thusproviding a differential clamping action.

After the sheet 20 is thus formed-here, into a leading-edge skinsection-the ram of the press israised, the screws l8 are loosened andthe section is removed 'endwise from the gripping jaws Figures 2 and 4show a slightly modified form of the preferred embodiment of the clamp.In this form, the ledge or thicker part, I9, is replaced by a shim l9corresponding to the thickness of the work sheet 20 and the standardscrews l8 are replaced by studs 33 provided with left and right- On thelower threaded portion ofeachof said studs is screwed a cylindrical nut34 carrying two diametrically opposed trunnions 35 on which is swivelledthe forked end 35 of a, handle in two parts 3'! fastened together bymeans of short bolts 38. The curved part of thefork 36 forms aneccentric or cam 39 allowing for aquick operation of the clamp after ithas been adjusted by turning the studs 33 with a screw-driver.

The handles 37 may be locked in their active position by means oflatches 40, pivoted as at 4| to a bracket 42 mounted on the side of thelower jaw l1. Each latch 40 swings in a slot 43, formed between the twoparts of each handle 31, when said handle r'neets a ramp M formed at thefree end of the latch.

Figure 3 shows diagrammatically the relative magnitude and directions ofthe four reaction forces Pl, P2, P3, and P4, which oppose the stretchingforce P when the clamp is closed and the machine operated. As in thisspecific embodiment angle a=100 and angle 5:80 and as the pressureapplied by the wedge is or IE on the upturned wing of the U-channel issubstantially greater than the pressure applied by the jaw I! or I Iunder the bottom of the U-channel,the following conditions obtain:

By. thus setting up four distinct clamping 'forces, and increasing theirmagnitudes; and by spreading these forces over a much larger area thaninsubsistent clamps, stress concentration at the point where the sheetleaves the clamp is avoided, thereby entirely obviating the tendency ofthe sheet to tear at this point. This tendency of the sheet to tear isfurther effectively precluded by having all the active surfaces of thelaws of the clamps perfectly smooth, as distinguished from the clamps ofthe kind in which said surfaces are roughened, or provided with smallteeth, in order to increase their gripping action.

It is to be understood that each of theopposite longitudinal edges ofthe work sheet is formed with angled borders,- or compound bends-heredisclosed as twice bent-merely in order to facilitate clamping andholding of thework sheet and in the ultimate product, these bends neednot appear.

It is to be noted that although the triple jaw clamp employed incarrying out the present invention,.when operated by the tighteningmeans disclosed, applies a differential pressure over different portionsof the compound bends of the angled borders of the work sheet, itapplies this pressure progressively, and over the entire, total surfaceof the pre-formed borders, with this pressure increasing most toward theextreme edges of the work sheet. Moreover, the stretching load is takenup, progressively, and in sequence, by four increasing,retaining-forces; by a first, rounded bend between the main portion ofthe work sheet and each of the angled borders; by the frictionindirectly created between the fixed jaw and one of the movable jaws ofthe clamp; by a second rounded bend forming the angle of each border;and by a second frictional reaction created directly between the fixedjaw and the other movable jaws of the clamp.

The foregoing describes in detail the stretch press employed in carryingout the ultimate or final forming of the sheet 20 into a hollow body andalso describes the novel clamping means (:0- operating with thepreformed opposed edges of the sheet 20 all of which forms the subjectmatter of the mechanical or means parent application Serial No. 464,357above referred to.

However, 'it is manifest that the structure shown'and described is notessential per se to the practice of the present method, but is solely anexample of the type of stretch press or apparatus especially suitable tothe final forming of the sheets as taught by the present method. Thepresent invention consists in preforming the opposed edges of a flatmetal sheet 20 to create a U- channel at each edge thereof projectingfrom and above the same surface of the sheet. The creation of each ofthese channels is accomplished by bending a terminal flange 20" from theedge of the sheet at an acute angle to the sheet so that the terminalflange 20" extends over the body of the sheet. A base flange 20 isformed by bending the sheet adjacent the terminal flange 20 to restsubstantially at right angles to the body of the sheet whereby theterminal flange 20 is positioned above the sheet to converge toward thesheet. These operations may be performed by any suitable means but ithas been found that a brake is especially adaptable for that purpose.Upon the completion of the formation of the U-channels, the relativelyflat sheet 20 has the base flange 20' projecting from each opposed edgethereof, said flanges being substantially parallel one to the other, andeach.

above the same surface of the sheet, is provided with a terminal flange20" converging inwardly toward the sheet. Thus the U-channels at thispoint in themethod face and open toward one another.

The sheet 20 is then roughly or partially preformed by bending itmedially between the U- channels at the edges of the sheet 20 to bringsaid channels into rather widely spaced positions upon the exterior ofthe roughly preformed hollow body, the base flanges 20' then resting insubstantially the same plane while the terminal flanges 2B" convergetowards the outer surface of the roughly or partially preformed body.

The body thus roughly or partially preformed is then slid or otherwiseplaced upon the press form block if as above described and the U-channels at its edges are clamped as hereinbefore set forth.

It will be again noted that the clamping force P is normal to the baseflange 20' while the clamping force P exerted on the terminal flange 20"is normal to that flange. It will also be observed that there are 4points of clamping or retaining pressure, viz.: P ,P ,P and P and thatthe clamping pressure P exceeds in value that of any of the otherclamping or retaining pressures and that the clamping pressure P exceedsthe value of the clamping or retaining pressures P and P The total ofthese pressures, P P P and P at all times exceeds the stretching forceor pressure P exerted on the inner surface of the partially preformedsheet 20.

Manifestly, the formation of the clamping or engaging zones at theopposed edges of the sheet 20 is such that only a minimum of material isrequired and yet the formation thereof is such that there is no sli pageor movement of these clamping or engaging zones during the stretching ofthe sheet 20 by internal pressure to its ultimate or final form. Thisnot only is a savin in metal but insures com lete dimensional accuracyin the finally formed hollow body which is vital to mass production andassembly.

Having thus disc osed the inventive concepts and several forms ofembodiment thereof, what is claimed is:

1. The method of forming hollow bodies from flat metal sheets consistingin creating clamping zones at the ed es of a flat sheet positionedsubstantially at right angles to the body of the sheet and projectingfrom and above the same surface of the sheet, roughly preforming thesheet by bending it medially between said clamping zones in a directionopposed thereto, clampingly engaging said zones by the applicationthereto of multiple clamping pressures progressively increasing in valuetoward the inner edge of each clamping zone, and finally forming thebody by applying a stretching pressure to the face of the sheet opposedto said angularly disposed clamping zones of a value less than theaggregate clamping pressure applied to each clamping zone.

2. The method of forming hollow bodies from a flat metal sheetconsisting in forming terminal flanges at the opposed edges of a sheetdisposed at acute angles to the sheet and each extending inwardly overthe body of the sheet, forming base flanges adjacent to each terminalflange by bending the sheet at right angles adjoining each terminalflange to position the terminal flange over the body of the sheet in aposition at acute angles to both the sheet and the base flange therebycreating channels at the opposed edges of the sheet opening toward eachother and projecting above the same surface of the sheet, roughlypreforming the sheet by bending it medially between said channels in adirection opposed to the channels to position the base flanges insubstantially the same plane, clampingly engaging said channelsexternally of the roughly preformed body by applying thereto clampingpressures progressively decreasing in value from the terminal flanges tothe inner extremities of the base flanges, and finally forming the sheetwhile the channels are clampingly engaged by applying pressureinternally thereof in a direction substantially normal to the plane ofthe base flanges of the channels.

3. The method of forming hollow bodies from a flat metal sheetconsisting in forming terminal flanges at the opposed edges of the sheetdisposed at acute angles to the sheet and each extending over the bodyof the sheet, forming a base flange adjacent to each terminal flange bybending the sheet at right angles to position the terminal flange overthe body of the sheet in a position at acute anglesto both the'sheet andthe base flange thereby creating channels at the opposed edges of thesheet opening toward each other and projecting above the same surface ofthe sheet, roughly preforming thesheet by bending it medially betweensaid channels in a direction opposed to the channels to position thebase flanges in substantially the same plane, clampingly engaging saidchannels by applying clamping pressure to the base flanges and to theterminal flanges thereof, the clamping pressures applied to the terminalflanges being greater than those applied to the base flanges, andsubjecting the roughly preformed sheet to an independent formingpressure on'the face thereof opposed to the face carrying said channels,while said channels are clampingly engaged as aforesaid.

4. The method of forming hollow bodies from a flat metal sheetconsisting in bending opposed edges of the sheet to form terminalflanges disposed at acute angles to the sheet and extending over thesheet, bending the sheet at right angles adjacent each terminal flangeto form a base flange thereby positioning each terminal flange over thebody of the sheet at an acute angle to both the sheet and its baseflange and thus creating channels at the opposed edges of the sheetopening toward each other and projecting above the same surface of thesheet, roughly preforming the sheet by'bending it medially between saidchannels in a direction opposed to the channels to position the baseflanges in substantially the same plane, clampingly engaging saidchannels by applying independent clamping pressures to the base flanges.and to the terminal flanges thereof, the clamping pressure applied tothe terminal flanges being greater than those applied to the baseflanges, and completing the formation of a hollow body by subjecting theroughly preformed sheet with its channels clampingly engaged to a finalforming pressure internally thereof of less value than the aggregatevalues of the clamping pressures applied to the terminal flange and baseflange of each channel, said forming pressure being from a sourceindependent of the several clamping pressures aforesaid.

HOWARD M. JAGER.

